The present invention relates to a method and a device for filling a fuel cell stack, including a plurality of fuel cells connected to one another in series, and at least one gas-intake and at least one gas-exhaust port, so that all or some portion of the individual cells in the series can be operated in parallel.
In order to attain sufficiently large voltages or currents, it is known to combine a plurality of fuel cells of the same kind to form a fuel cell stack. For this purpose, the fuel cells are connected in parallel and/or in series and have a common intake port and exhaust port. If the fuel cell stacks are not supplied with pure fuel gas, then a fuel-gas concentration gradient results on the anode side from the gas-intake port to the gas-exhaust port. For example, this is the case when operating with a series-connected methanol reformer for generating hydrogen. In this case, the composition of the anode gas at the gas intake is about 55% H2, 22% N2, 22% CO2 and 1% O2, whereas the H2 concentration at the gas exhaust can have decreased by up to 10%. Then, the volume percentages of the other gases are correspondingly higher. To operate with a high efficiency factor, i,e., as much hydrogen as possible should be converted in the fuel cells, the fuel gas is passed through all of the available fuel cells using two or more fuel cell stacks connected in series, so that the hydrogen concentration at the gas exhaust is ideally zero. At the same time, the power output of the fuel cell decreases with the fuel gas concentration at the anode, i.e., the output of the fuel cells in front of the gas exhaust is lower than that of the first fuel cells directly behind the gas intake.
To operate with high power output, all of the cells and stacks can be supplied with gas simultaneously; however, the concentration of hydrogen in the exhaust gas is higher than in the series-connected fuel cell stacks first described.
When using fuel cells in motor vehicles, it is first of all desirable that as much power as possible be available for accelerating and driving at high speed, the efficiency of the energy conversion being of secondary importance in this case. On the other hand, it is desirable during part-throttle operation, e.g., while driving at constant speed or driving in the city without accelerating hard, to achieve as high an energy conversion efficiency as possible, to increase the driving range with a full tank and lower the operating costs.
Therefore, the present invention is based on the technical problem of providing a method and a device for filling fuel cell stacks, which fulfill the two above-mentioned requirements satisfactorily.
By providing a gas-intake port having a closable gas flap valve to each fuel cell or a group of fuel cells, the gas-flap valve being operable by an actuating signal, it is possible to switch between the high-efficiency and the high-output operating modes. The fuel cell stack can be operated predominantly in a high-efficiency mode, i.e., dosed gas-flap valves; and then the gas-flap valves can be opened to supply individual fuel cells or groups of fuel cells with additional fuel gas when a high power output is required, e.g., when accelerating.
The present invention provides an improved fuel cell stack comprising a plurality of fuel cells connected in series including a first fuel cell having at least one main-gas intake port and a last fuel cell having at least one main gas-exhaust port; and a supplemental gas intake port having a closable valve disposed between the main gas intake port and the main gas-exhaust port.
The invention also provides an improved method for filling a fuel cell stack, including a plurality of fuel cells connected in series including a first fuel cell having a main gas-intake port and a last fuel cell having a main gas exhaust port, and at least one supplemental gas-intake port having a closable valve operable by an activating signal, which comprises a) generating an actuating signal as a function of the fuel cells power output; and b) opening and closing the supplemental gas intake port valve in response to the actuating signal.
The present invention further provides an improved method for filling a fuel cell stack, including a plurality of fuel cells connected in series including a first fuel cell having a main gas-intake port and a last fuel cell having a main gas-exhaust port, and at least one supplemental gas-intake port having a closable valve operable by an actuating signal, which includes: a) generating an actuating signal as a function of the fuel stacks power output requirement; b) opening and closing the supplemental gas-intake port valve in response to the actuating signal; and c) measuring pressure-drop across the fuel cell stack and generating the actuating signal as a function of a threshold value for the pressure drop.